The present invention relates to a check valve for installation in various pipeline systems, and more particularly to a check valve assembly of the quick shutoff type equipped with an auxiliary valve of the slow closure type for protecting the pipeline system from waterhammer caused by pump failure.
In the event that a hydraulic pump in a pipeline is suddenly stopped due to cutoff of the electric power supplied thereto, quick shutoff of the pipeline by a conventional check valve will cause dangerous waterhammer if the valve is slammed after the occurrence of large flow. For the purpose of eliminating the occurrence of such dangerous waterhammer, there have been proposed two sorts of check valves, one of which is the quick shutoff type adapted mainly for use in a pipeline of small or medium diameter, and the other of which is the slow closure type adapted mainly for use in a pipeline of medium or larger diameter. The check valve of the quick shutoff type includes a valve body which is arranged to be quickly closed under load of a relatively strong spring for decreasing waterhammer applied thereon. Such arrangement of the spring results in an increase of valve resistance in the pipeline. Even if the valve body was closed in an ideal manner without any delay in time, pressure head at the outlet of the check valve would be increased to a value defined by the difference between an actual lift and a pressure head immediately before the valve is closed. This means that the pressure head becomes larger in application of the check valve to a long pipeline.
The check valve of the slow closure type includes a primary valve arranged to permit a large amount of fluid passing therethrough, and an auxiliary valve mounted on the primary valve and operatively connected to a hydraulic dash pot. When the primary valve has been closed, the auxiliary valve is maintained in an open position by the hydraulic dash pot to restrain rise of the pressure head caused by closure of the primary valve and is gradually closed under control of the hydraulic dash pot to prevent the occurrence of waterhammer. The primary valve includes a valve housing formed therein with a cylinder bore, a valve shaft rotatably mounted within the valve housing, and a primary valve plate of the swing type rotatably mounted on the valve shaft to open and close the cylinder bore and being formed at a portion thereof with a through hole of relatively large diameter for permitting the flow of hydraulic fluid passing therethrough. The hydraulic dash pot is mounted on the valve housing at the outside of the same and is operatively connected to one end of the valve shaft to control opening and closing operations of the auxiliary valve. The auxiliary valve includes a secondary valve member mounted on the primary valve plate to open and close the through hole in the primary valve plate.
In such arrangement of the check valve assembly as described above, the primary valve plate is closed by reverse flow of the hydraulic fluid applied thereon. For this reason, even if the through hole in the primary valve plate was formed relatively large in diameter to restrain the pressure head acting on the primary valve plate, it would be unable to completely prevent the occurrence of waterhammer caused by closure of the primary valve plate. Furthermore, when the primary valve plate is slammed, the secondary valve member is applied with a large amount of reverse flow of the hydraulic fluid. As a result, it is required to enlarge the displacement capacity of the hydraulic dash pot. This means that the check valve assembly is inevitably complicated in construction and becomes costly in manufacturing. Additionally, in use of the check valve assembly for a long period of time, there will occur frictional defacement of the valve shaft on the valve housing and leakage of the hydraulic fluid across a sealed portion of the valve shaft.